Interception is precipitation that does not reach the soil, but is instead intercepted by the leaves and branches of plants and the forest floor. The intercepted water generally evaporates and leads to loss of that precipitation for the drainage basin. ~ excess precipitation ~ loss
In HMS part of sub-basin properties. Simple Canopy Gridded Canopy HMS “gridded” implies a GIS type interface This course does not directly use gridded methods, although introduced in last module.
All precipitation intercepted until storage capacity satisfied. Excess precipitation then directed to surface (depression) storage if any. Then excess to runoff component. Also considers potential evapo-transpiration (PET) as part of the hydrologic cycle.
Sophisticated hydrologic abstraction Likely uncommon in typical engineering hydrological applications, esp. because of the PET feedback Utility in “scientific investigation” Measurements are non-existent While the process undoubtedly occurs, would not be commonly used in Texas, except perhaps East Texas Piney Woods
Depression storage. The volume of water contained in natural depressions in the land surface, such as puddles. (After Horton, 1935, p. 2) In the Green-Ampt model, water ponds at non-zero depth; hence depression storage is arguably important for such infiltration models. The interaction of depression storage and infiltration is the basis of Hortonian overland flow
In HMS part of sub- basin properties. Simple Surface Gridded Surface
Initial storage (percent of maximum) Maximum storage (depth) Storage is satisfied. Excess can become runoff.
As a process diagram: Loss Model(s) Precipitation Depression Storage Excess Precipitation Interception Storage Infiltration Evapotranspiration Sub-basin properties Meterologic properties
Reservoir A pond, lake, or basin, either natural or artificial, for the storage, regulation, and control of water. ▪Regulated reservoir ▪Outflow controlled by moveable gates and valves. ▪Head, and valve settings determine outflow. ▪Unregulated reservoir. ▪Outflow controlled by fixed weirs and orifices. ▪Head and constructed weir height determine outflow.
In HEC-HMS reservoirs (and detention basins) are treated as a hydrologic element in the basin model
Accounts for storage Flows are “routed” through a reservoir Level pool routing Orifice flow Weir flow
Pond with storage, orifice and weir flow. Orifice flow; energy loss model Weir flow; critical depth model Image from ftp://ftp.crwr.utexas.edu/pub/outgoing/Robayo/HECHMS.../HEC-HMS.ppt
Storage Representations Storage vs. Discharge Storage vs. Elevation Surface Area vs. Elevation Discharge Representations Spillways, Weirs Orifices, Sluice gates Pumps Dam Breach Image from ftp://ftp.crwr.utexas.edu/pub/outgoing/Robayo/HECHMS.../HEC-HMS.ppt
The storage relationships are usually developed external to HEC-HMS Like rainfall and external hydrographs, use external tools to develop the storage- discharge relationships
Example 5 – Illustrate Reservoir Storage Data Entry Ash Creek Watershed Use the GA runoff generation model ▪Will use canopy storage and surface storage to illustrate the effects of these components. Pretend we will place a small detention facility at the outlet ▪Develop the storage-discharge curves in Excel ▪Enter into HEC-HMS, examine effects.
Storage in HEC-HMS is of two types: Abstraction: Canopy and Depression Hydrologic/Hydraulic: Reservoir Abstraction storage is a sophisticated concept, hard to estimate parameters for engineering practice – its is uncommon. Reservoir storage is common, if not fundamental in watershed models Detention facilities BMPs
Example 5 illustrates the data entry activities associated with both kinds of storage.